Radiation detectors that employ gas ionization can be used to detect various types of radiation, such as neutron radiation, beta particle radiation, gamma particle radiation, or X-ray radiation. A gas ionization radiation detector includes a gas chamber filled with a fill gas such as helium-3. The gas chamber also includes a thin wire that acts as an anode. The wire is suspended within a tube that acts as a cathode. The tube and the thin wire create an electric field within the gas chamber. The gas ionization radiation detector relies on the principle that a charged particle that travels through the gas will interact with the fill gas and produce ions and electrons (e.g., ionization). The electrons will drift towards the thin wire and generate avalanches that produce further electrons and ions. These electrons and ions can be detected at the wire. In this manner, the gas ionization radiation detector detects radiation that travels through the gas chamber.
Helium-3 is a particularly efficient fill gas for detecting neutron radiation due to the large neutron capture cross section of the helium-3 isotope. As a neutron travels through the gas chamber, the neutron interacts with a helium-3 atom to create a proton and a triton. These charged particles ionize other atoms of helium-3 to produce electrons and ions. Such helium-3 gas chamber detectors have been used in downhole oil and gas field applications to detect neutrons for a variety of different measurements, such as formation neutron porosity and formation “hydrogen index” or “sigma” (e.g., with a pulsed neutron source).
Although helium-3 is an efficient gas for detection of neutrons, helium-3 gas is becoming more expensive and less available due to scarcity and increased demand. Conventional alternative technologies, such as tubes lined with boron or gas chambers filled with boron tri-fluoride, fail to match the efficiency of helium-3 for neutron detection. The problems with such alternative technologies are compounded in downhole applications, where available space is limited and the environment includes high pressures, high temperatures, substantial mechanical vibrations, and strong mechanical shocks.